High frequency amplifier



R. A. BRADEN HIGH FREQUENCY AMPLIFIER April 25, 1939.

Filed Nov. 19, 1935 2 Sheets-Sheet 1 E1.2 4a .96 44 as 45 42 v INVENTOR Rene H. Braden Wiinesa A ril 252 1939. R. A. BRADEN HIGH FREQUENCY AMPLIFIER 10 10 10 10 2 7r (freq. in Cycles 092- sec.) 2 7r (freq. in cycles p er sec.)

100 FREQ. IN A. c.

, INVENTOH Rene H.Braden I Witness:

H'I'TOPNEY Patented Apt. 25, 1939 UNITED STATES PATENT OFFICE HIGH FREQUENCY AMPLIFIER Delaware Application November 19, 1935, Serial No. 50,520

7 Claims.

My invention relates to high frequency amplifiers and particularly to amplifiers adapted to amplify picture signals or the like.

In the past it has been found difficult to obtain the desired amplification at the upper frequency limit of an amplifier designed for amplifying picture signals in a. television system. If a given amplifier circuit is designed to provide high gain at the upper frequency limit it does not have a fiat frequency response curve.

Also, such an amplifier generally has an undesired delay or phase shift characteristic at the higher frequencies which distorts a picture signal.

It is an object of my invention to provide an improved amplifier in which the high frequency range is extended and which has a substantially fiat frequency response curve.

It is a further object of my invention to provide an improved amplifier giving substantially constant delay over the entire frequency range.

In practicing my invention I employ two types of amplifier stages, one of these stages having gain and delay characteristics which are substantially complementary to the gain and delay characteristics of the other amplifier stage. Specifically, one amplifier stage is of the type containing a so-called peaking coil in series with the plate resistor while the other amplifier stage 30 is of the type containing a resonating coil in series with the interstage coupling condenser.

Other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying 35 drawings in which Figure l is a circuit diagram of an amplifier embodying my invention,

Fig. 2 is a circuit diagram of another embodiment of my invention,

40 Figs. 3 and 4 are circuit diagrams showing variations of one of the amplifier stages illustrated in Fig. 2,

Figs. 5 and 6 are curves showing the gain and delay characteristics respectively of the amplifier 45 shown in Fig. and

Fig. 7 shows the gain curves for the amplifier stages illustrated in Figs. 2, 3, and 4.

Referring to Fig. 1, the invention is shown applied to a television receiver which includes a cathode ray receiver tube In the specific embodiment which is illustrated, the amplifier comprises two unlike amplifier stages 2 and 3, the first amplifier stage including an electric discharge tube 4 and the second amplifier stage in- 55 eluding an electric discharge tube 6, both tubes sistor 2 preferably being of the screen grid type such as the RCA-36.

The electric discharge tube 4 includes the usual cathode '1, control grid 8 and plate 9, the control grid 8 being biased negatively by means of the conventional self-bias resistor II. A suitable operating voltage is applied from a voltage source (not shown) to the plate 9 through a plate resistor I2 and an inductance coil l3 which will be referred to as a peaking coil. 10

The second electric discharge tube 6 likewise has the usual cathode l4, control grid l6 and plate I1 and, as in the case of the first tube, the control grid I6 may be maintained at a suitable negative bias by means of the conventional 15 self-bias resistor I8, the biasing voltage being applied to the grid l6 through a grid resistor [9.

A suitable operating potential is applied to the plate electrode I! from any suitable source (not shown) through a plate resistor 2|, an inductance 20 coil 22 and a variable damping resistor 23 connected in series. As will be pointed out later, the inductance coil 22, which will be referred to as a resonating coil, is in a diiferent position in the amplifier stage than is the peaking coil 25 [3 whereby it functions somewhat differently although it, like the peaking coil, is employed for the purpose of extending the amplifier range at the high frequency end.

The two amplifier stages are coupled in cascade, the output circuit of the first amplifier stage 2 being coupled to the input circuit of the succeeding amplifier stage 3 through a coupling condenser 24.

The output circuit of the amplifier stage 3 is coupled to the control grid 25 of the cathode ray tube I, this coupling being through a coupling condenser 21 connected between the control grid 26 and a variable tap 28 on the plate re- The control grid 26 is maintained at a suitable negative bias by means of a biasing battery 29 or the like,

In the amplifier stage 3 the coil 22 resonates with the output capacity of the tube 6. It would also resonate with the input capacity of the cathode ray tube (there being a series circuit including the output capacity of tube 6, coil 22, coupling condenser 27 and the input capacity of cathode ray tube I) except that the input capacity of tube I is substantially short-circuited by the comparatively low resistance resistor 2|.

It will be noted that only the voltage across the resistor 2| is applied to the succeeding tube, not the voltage across both the resistor and the coil as in the preceding stage.

In some cases it may be desired to employ a direct current connection between the variable tap 28 and the control grid 26 in place of the coupling condenser 21 for the purpose of making the cathode ray tube grid bias vary with change in direct current flow through the plate resistor 2|. In that event, of course, the cathode 3| of the cathode ray tube is maintained at some suitable potential other than ground potential. In some instances the damping resistor 23 may be omitted although, in general, it is necessary in order to avoid too large a peak in the frequency response curve.

The second amplifier stage illustrated in Fig. 1 is described. and claimed in my copending application, Ser. No, 759,645, filed Dec. 29, 1934, Patent No. 2,084,475, and assigned to the Radio Corporation of America. v

In the first amplifier stage 2, the input capacity of the tube 6 may be considered in parallel with the output capacity of the tube 4 since the impedance of the coupling condenser 24 is very low at high frequencies. While the peaking coil |3 may be said to resonate with these two tube capacities in parallel at the upper frequency limit of the amplifier, there is no sharp resonant point since the plate resistor I2 is included in series in the resonant circuit. In the first amplifier stage 2, the voltage appearing across the peaking coil I3 is impressed upon the input circuit of the tube 6, while in the second amplifier stage 3 the resonating coil 22 merely causes an increase in the current flow through the plate resistor 2| whereby there is a resulting increase in the voltage drop across this resistor. Instead of considering the peaking coil l3 in the first amplifier stage as a coil which resonates with tube capacity, it may be considered merely as a reactance element in the plate circuit, the impedance of which increases with increase in frequency to compensate for loss due to tube capacities to ground.

As previously mentioned, the two amplifier stages are so designed with respect to each other that both their overall gain characteristic and their overall delay characteristic are substantially flat over the entire frequency range. This will be better understood by referring to the curves shown in Figs. 5 and 6. Referring to Fig. 5, the gain of the first amplifier stage 2 is shown by the curve A and the gain of the second amplifier stage 3 is shown by the curve B. As shown by the curve B, it has been possible to hold up the gain better at the high frequencies by means of the second amplifier stage than by means of the first amplifier stage. However, the curve B contains an undesirable dip.

The amplifier stage 2 is so designed that its peak occurs at substantially the same frequency as the dip in the response curve of the other amplifier stage and the magnitudes of the peak and dip in the two response curves are made such that, when the gains of the two amplifiers are multiplied, the overall frequency response is substantially flat as shown by the curve C.

Referring now to Fig. 6, the delay characteristic of the amplifier stage 2 is shown by the curve A while the delay characteristic of the amplifier stage 3 is shown by the curve B. It will be seen that, as in the case of the gain curves, the curve A has a peak where the curve B has a dip and that by suitably designing the two amplifier stages their delay characteristics may be made such that they will aid to give a substantially fiat overall delay characteristic as indicated by the curve C.

In Figure 1, specific values in ohms, millihenries and micromicrofarads are indicated for the specific embodiment of the invention for which the curves shown in Figs. 5 and 6 are drawn. It will be understood, however, that various other circuit values may be used, it only being necessary that one amplifier stage be properly designed with respect to the other amplifier stage. It will also be understood that in place of a single amplifier stage employing a peaking coil a plurality of such stages may be employed as their overall amplification will have the same general characteristic as a single stage. Likewise, the amplifier stage having the resonating coil may be replaced by a plurality of similar stages. In the claims, therefore, the expression an amplifier stage is meant to include one or more amplifier stages of the same type.

Referring to Fig. 2, this embodiment of my invention, like the embodiment illustrated in Fig. 1, comprises an amplifier stage 36 of the resonating coil type including electric discharge tube 34 and an amplifier stage 3'| of the peaking coil type including electric discharge tube 35. The order in which the two stages are connected in cascade is immaterial, but in the particular embodiment illustrated, the amplifier having the peaking coil is the last amplifier stage.

The main difference between the amplifier shown in Fig. 2 and that shown in Fig. 1 is that in the resonating coil type of amplifier stage 36, two resonating coils 38 and 39 are employed in place of a single resonating coil, the plate voltage being supplied to the tube 34 through the usual plate resistor 4| to a point between the two coils. With this circuit arrangement, the resonating coils 38 and 39 form a resonant circuit with the output capacity of the tube 34 and the input capacity of tube 31 which resonates at the high frequency end of the operating range of the amplifier. This resonant circuit includes in series the output capacity of tube 34, the coils 39 and 39 and the input capacity of tube 35. Because of coil 39, the resistor 4| does not have any appreciable short-circuiting action on the input capacity of tube 35. The series circuit just traced also includes a coupling condenser 42 which has very low impedance at the higher picture frequencies and radio frequency choke coils 43 and 45 which likewise have very low impedance at the higher picture frequencies.

With the specific form of amplifier stage indicated at 36, it is possible to extend the amplifier range somewhat further at the high frequency end than is possible where a single resonating coil is used as in Fig. 1. It also provides a certain flexibility in design in that the dips and peaks of the response curves of the two amplifier stages may be changed in amplitude and location in the frequency spectrum by changing the value of either one or both of the resonating coils.

It is generally preferred to include a variable damping resistor 44 between the coils 38 and 39 and in series therewith. By adjusting the value of this resistor, the amplitude of the dips and peaks in the response curves may be given the desired values.

When the damping resistor 44 is employed, the plate resistor 4| may be connected between the coil 39 and the resistor 44 as shown in Fig. 2, or between the resistor 44 and the coil 39 as shown in Fig. 3. In some cases, it may be preferred to connect the plate resistor 4| between the coil 39 and the coupling condenser 42 as shown in Fig. 4. Each time the point of connection of the plate resistor 4| is changed, the response curves of the amplifier stage are changed slightly as indicated by the curves in Fig. '7. The curves A, B and C in this figure are the curves for the circuits in Figs. 2, 3 and l, respectively. Thus, it will be seen that shifting this point of connection may aid considerably in making the gain and delay curves of the two amplifier stages complementary.

In Fig. 2, for the purpose of specific illustration, it has been assumed that electric discharge tube 34 of the amplifier stage is the second detector tube in a television receiver. Therefore, the radio frequency choke coils 43 and 45 have been shown included in the circuit for the purpose of filtering out the carrier frequency. As previously stated, the inductance of these coils is so small that they have substantially no effect upon picture signals.

From the remarks made in connection with Fig. 1, it will be understood that if the amplifier stage 38 is provided solely for amplification rather than detection, a preceding stage similar to the amplifier stage 3'! may be employed for the purpose of detection or, of course, merely for amplification if desired. In that event, the overall gain and delay curves of the two peaking coil stages would be made complementary to the delay and gain characteristics of the resonating coil stage.

In the claims, the term amplifier stage includes a stage such as the amplifier stage 36 in which a signal is detected as well as amplified.

From the foregoing description, it will be apparent that various modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations may be imposed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim as my invention:

1. In a multistage amplifier, an amplifier stage comprising an electric discharge tube having a plate electrode, means including a coupling condenser for coupling said plate electrode to a succeeding electric discharge tube, a plate resistor and an inductance coil connected in series between a source of plate voltage and a point between said plate electrode and said coupling condenser, an amplifier stage comprising a second electric discharge tube having a plate electrode, means including a coupling condenser for coupling said second plate electrode to a succeeding electric discharge tube, said coupling means including an inductance coil connected in series with said second coupling condenser and said second plate electrode, and means for applying an operating voltage to said second plate electrode, said two amplifier stages being coupled in cascade and being so designed that at the high frequency end of the operating range the gain and delay characteristics of one stage are substantially complementary to the gain and delay characteristics of the other stage.

2. An amplifier comprising an electric discharge tube having a plate electrode, a second succeeding electric discharge tube, means for coupling said plate electrode to said succeeding electric discharge tube, said coupling means including a first inductance coil and a second inductance coil connected in series between said plate electrode and said succeeding tube, there being substantially no inductive coupling between said coils, said coils having such values of inductance that they resonate with the capacity of said tubes at the high frequency end of the operating range of the amplifier, and a plate resistor connected between a source of plate voltage and a point between said two coils.

3. In combination, an amplifier as defined in claim 2 wherein said second electric discharge tube has a plate electrode, a circuit for coupling the plate electrode of said second tube to a succeeding electric discharge device, an inductance coil and a resistor connected in series between a source of plate voltage and a point in said coupling circuit, said last coil having an inductance of such value that it increases the gain of said second tube at the high frequency end of the operating range, the two amplifier stages formed by said two tubes being so designed that the gain and delay characteristics of one amplified are substantially complementary to the gain and delay characteristics of the other amplifier.

4. The invention according to claim 2 characterized in that said coupling means includes a resistor connected between said two inductance coils and in series therewith.

5. In combination, a plurality of amplifier stages connected in cascade, each of said amplifier stages comprising an electric discharge tube having a plurality of electrodes including an output electrode, there being unavoidable capacity between said output electrode and ground, said capacity including interelectrode capacity, an output circuit connected to the output electrode of one of said tubes, said circuit including a plate resistor and an inductance coil, the output from said one tube being taken from across said plate resistor and said inductance coil, a second output circuit connected to the output electrode of another of said tubes, said second circuit including an inductance coil adjacent to the output electrode of said other tube and a resistor in series, said last inductance coil having such inductance that it resonates 'with said unavoidable capacity between the output electrode of said other tube and ground at the high frequency end of the amplifiers frequency range, the output from said other tube being taken from across said last resistor only, both of the amplifier stages comprising said tubes having the characteristic that at the high frequency end of the operating range the gain and delay vary with frequency, and said stages being so designed that the said variations in one stage are substantially complementary to said variations in the other stage.

6. In combination, a plurality of amplifier stages connected in cascade, each of said amplifier stages comprising an electric discharge tube having a plurality of electrodes including an output electrode, there being unavoidable capacity between said output electrode and ground, said capacity including interelectrode capacity, an output circuit connected to the output electrode of one of said tubes, said circuit including an inductance coil adjacent to the output electrode of said one tube and including a resistor in series with said inductance coil, said inductance coil having such inductance that it resonates with said unavoidable capacity between the output electrode of said one tube and ground at a frequency in the high frequency end of the amplifiers frequency range such that the frequencygain curve of the amplifier stage including said one tube has a dip on the low frequency side of said resonant point, the output from said one tube being taken from across said resistor only, a second output circuit connected to the output electrode of another of said tubes, said second circuit including a plate resistor and an inductance coil, the output from said other tube being taken from across said plate resistor and said inductance coil, said last inductance coil havin such inductance with respect to the unavoidable capacity of said other tube that the frequencygain curve of the amplifier stage comprising the said other tube has a peak at approximately the same frequency as that at which the dip in said first frequency-gain curve occurs, whereby the overall frequency-gain characteristic of said amplifier stages is approximately flat.

7. In combination, a plurality of amplifiers of diiTe-rent types connected in cascade, each of said amplifiers comprising an amplifier'stage which includes an electric discharge tube having a plurality of electrodes including an output electrode, there being unavoidable capacity between said output electrode and ground, said'capacity including interelectrode capacity, an output circuit connected to the output electrode of a tube in one of said amplifiers, said circuit including a plate resistor and an inductance coil, the output from said last tube being taken from across said plate resistor and said inductance coil, a second output circuit connected to the output electrode of a tube in another of said amplifiers, said'second circuit including an inductance coil adjacent tothe output electrode of said last-mentioned tube and a resistor in series, said last in ductance coilhaving such inductance that it resonates with said unavoidable capacity between the output electrode of said last-mentioned tube and ground at the high frequency end of the amplifiers frequency range, the output from said last-mentioned tube being taken from across said last resistor only,- b'oth of the amplifiers comprising said two last-mentioned tubes having the characteristic that at the high frequency end of the operating range the gain and delay vary with frequency, and said amplifiers being so designed that the said variations in one amplifier are substantially complementary to said variations in the other amplifier.

RENE A. BRADEN. 

